1. FIELD OF THE INVENTION
The present invention relates to photomask blanks and photomasks for use in the production of semiconductors or the like.
2. DESCRIPTION OF THE PRIOR ART
In the production of semiconductors such as integrated circuits (IC) or large-scale integrated circuits (LSI), elements are produced by optical or electron beam lithography wherein a photomask formed with a chromium layer is used as a hard mask plate with high-resolution in order to form a circuit image on a silicon wafer with high precision. As such a photomask, it is common to employ a single layer type photomask comprising a single chromium masking layer formed on a transparent substrate, or a double layer type photomask wherein a layer of chromium oxide is formed for antireflection on the single layer type photomask to reduce the reflectance of the single layer type photomask which usually has a high surface reflectance.
For the preparation of a photomask used for the production of semiconductors from a photomask blank, a photo-sensitive or electron-sensitive resist layer is coated on the photomask blank, and then a predetermined pattern is exposed by an exposure apparatus, followed by development treatment of the resist, and then the exposed chromium layer portions are subjected to etching to obtain a desired pattern. Therefore, the chromium masking layer as a masking material is required to have excellent etching properties so that it is capable of forming a fine or minute pattern.
Further, the chromium masking layer as a masking material is required to have, in addition to the optical properties and etching properties, good durability, particularly chemical durability against a cleaning solution such as a strong acid, and good adhesion to the glass substrate. Japanese Unexamined Patent Publication No. 31336/1983 proposes to introduce a nitrogen component into the chromium masking layer to improve the chemical durability. However, the durability is inadequate against a strong acid cleaning solution represented by e.g. hot concentrated sulfuric acid. Further, it has been proposed to reduce the etching speed by introducing a carbon component into the masking layer (Japanese Unexamined Patent Publication No. 151945/1982, and U.S. Pat. No. 4,530,891). However, this proposal has drawbacks such that chromium carbide has poor chemical resistance and tends to lower the adhesion to the glass substrate.
On the other hand, a chromium oxide layer as an antireflection layer usually has a drawback that the etching speed is usually slower than that in the case of the chromium layer as the masking layer, and, as shown in FIG. 11, an overhang 50 is likely to form in the cross section of the pattern, whereby the precision for a fine pattern tends to deteriorate. Namely, if the overhang is formed along the pattern edges in a thickness of from 200 to 300.ANG., it is susceptible to breakage, and in various cleaning operations during the preparation or use of the mask, discontinuous burrs or mouse nip like defects are likely to form along the pattern edges, whereby the precision of the pattern tends to deteriorate. Further, microscopically, the overhang along the periphery of the masking pattern means that the optical density along the pattern edges tends to have a gradient, whereby the dimensional value at the time of transfer tends to be readily affected by the exposure condition. It has been proposed to introduce nitrogen into the chromium oxide layer to eliminate such drawbacks (see Japanese Unexamined Patent Publication No. 104141/1972). However, the composition of the layer is not still fully satisfactory in view of the degree of overhang, and a further improvement is desired.